1. Field of the Invention
The present invention relates to a roller mill for grinding particulate material such as cement raw materials, cement clinker, coal and similar materials, said roller mill comprising a substantially horizontal grinding table and a set of rollers rotating about a vertical shaft, said set of rollers comprising a number of rollers rotatable about separate roller axes and being connected via a roller bearing and a roller shaft to the vertical shaft, and said set of rollers being configured for interactive operation with the grinding table for application of pressure to the particulate material.
2. Background of the Art
A roller mill of the aforementioned kind is known, for example, from the UK patent No. 601,299. This known mill is designed so that the set of rollers rotate in one direction and so that the grinding table rotates in the opposite direction so as to increase the capacity of the mill. According to the above-mentioned patent publication, the rollers are connected to the vertical shaft via a crank-like connection where each roller is supported by a stationary crank which protrudes centrally into the roller. In the publication there is no detailed mention about how the roller is supported on the crank, but based on previous knowledge of roller mills this is most likely achieved either by means of a slide bearing or a rolling bearing provided in the roller itself. With reference to FIG. 1, and as defined in the introduction, the roller bearing for each roller is influenced, during the operation of a roller mill, by the reactions Fg,1 and Fg,2 from the grinding force Fg which occurs in the grinding zone between the roller and the grinding table. Also a gyro moment Mgyro will be generated about the centre of mass of each roller in the plane containing the centre axis of the roller, said gyro moment will result in the reaction forces Fgyro,1 and Fgyro,2 on the roller bearing. The magnitude of this gyro moment and hence of the reaction forces depend on the moment of inertia of the roller and its rotational speed about its separate roller shaft and on the rotational speed of the set of rollers about the vertical shaft. As is apparent from FIG. 1, the innermost part of the bearing, i.e. that part of the bearing which is located closest to the vertical centre shaft will be unilaterally impacted by the reaction force Fgyro,2 and by a reaction contribution Fg,2 from the grinding force. Hence, the total load imposed upon this part of the bearing may be quite substantial, resulting in early-stage wearing-down and/or breakdown of the bearing.